It has been proposed to use a patterned adhesive in various electronic parts, for example, for bonding a semiconductor element to a lead frame, bonding a semiconductor element to a circuit board, or bonding circuit boards to each other. Such an adhesive is generally obtained by applying a photosensitive resin composition to a semiconductor element, lead frame, circuit board, or the like as one adherend to first form a coating film of the composition, exposing the coating film to light through a photomask, and then developing the coating film to impart a given pattern thereto. Thereafter, the other adherend is superposed on the patterned adhesive to bond the adherends to each other by hot pressing.
In general, conventional adhesives such as the above one soften in hot pressing and penetrate into rugged surfaces of adherends to produce an anchoring effect. Thus, an adhesive strength is obtained.
However, such an adhesive does not have sufficient flowability in hot pressing although it softens, because it has a crosslinked structure formed during pattern formation through exposure and development. This adhesive hence penetrates insufficiently into the rugged surface of an adherend. As a result, vacant spaces remain on part of the bonding surface and a sufficient adhesive strength cannot be obtained.
Incidentally, recent electronic parts contain electronic elements in which the wirings, terminals, and other components have various shapes. The bonding surfaces of these components are not merely flat but rugged, and such rugged bonding surfaces tend to become more complicated and finer.
Consequently, use of adhesives such as the above-described one in bonding electronic elements results in bonding failures.